1. Field of the Invention
The present invention relates to multi-layer laminates of microporous films, a method for preparing same and articles prepared therefrom.
2. Background Information
Porous and microporous polymer films are well known in the art. The process of making these films to produce desirable properties usually involves stretching a film and this stretching process causes the film to become thinner.
There are many uses for microporous polymer films that are of a thickness that would require non-producible extrudates with thick cross-sections from the extruder before stretching. These films are not producible due to heat transfer, phase separation and precipitation limitations. To obtain the required film thickness layers of thin film can be laminated together.
Traditional bonding of porous polymer materials is done with either bonding agents or temperature/pressure techniques. Bonding agents or adhesives penetrate a significant distance into the pore network of the material thereby mechanically locking the layers in place. Temperature/pressure methods involve techniques of calendering, point bonding with a patterned nip, ultrasonic welding and the like. These methods generally alter the porosity of the layers at the interfaces and thus adversely affect the internal porosity and void volume of the laminated film. These methods can also have disadvantages of inclusion of non-polymer film material, contamination by foreign material, non-continuous interlayer bonding and inadequate inter-layer bonding. These boundary layer interruptions affect the utility of the layered film as a continuous, non-blocking and uniform gradient throughout the laminate is often desired.
U.S. Pat. No. 4,650,730 (Lundquist et al.) discloses a multiply sheet product useful as a battery separator having at least two plies each in the form of a microporous sheet of predetermined length and breadth and a thickness of less than 10 mils. The multiply sheet product is formed in conventional manners such as by coextensively overlaying at least one first sheet with at least one second sheet and subjecting them to nip rollers, calendering or the like. Alternatively, the sheet product can be formed by coating a second sheet with a first composition, normally of polymer and plasticizer, and then removing the plasticizer.
U.S. Pat. Nos. 4,385,093 and 4,478,665 (Hubis) disclose a multi-component, porous polytetrafluoroethylene (PTFE) article. Components of PTFE containing a liquid lubricant are placed in intimate contact, dried to remove the liquid lubricant and then stretched in one or more directions. The intimacy of contact between components can be enhanced by applying a compressive force such as by using a press or a set of calender rolls for contact.
PTFE exhibits cold flow properties and calendering PTFE not only reduces the thickness of the calendered sheet but also causes PTFE layers to cold weld. This cold welding or cold flowing causes the bonding of this invention.
U.S. Pat. No. 4,187,390 (Gore) and ICI Americas Inc. Technical Bulletin "The Processing of PTFE Coagulated Dispersion Powders," pp. 1-4, describe forming PTFE through compression or shear forces.
U.S. Pat. No. 4,247,498 (Castro) discloses microporous polymers characterized by a relatively homogeneous, three-dimensional cellular structure having cells connected by pores of smaller dimension. The microporous polymers are prepared from thermoplastic polymers by heating a mixture of the polymer and a compatible liquid to form a homogeneous solution, cooling the solution under non-equilibrium thermodynamic conditions to initiate liquid-liquid phase separation, and continuing the cooling until the mixture achieves substantial handling strength.
U.S. Pat. No. 4,539,256 (Shipman) discloses a microporous sheet material characterized by a multiplicity of spaced randomly dispersed, equiaxed, non-uniform shaped particles of the thermoplastic polymer, adjacent thermoplastic particles connected to each other by a plurality of fibrils of the thermoplastic polymer. The sheet materials are prepared by melt blending crystallizable thermoplastic polymer with a compound which is miscible with the thermoplastic polymer at the melting temperature of the polymer but phase separates on cooling at or below the crystallization temperature of the polymer, forming a shaped article of the melt blend, cooling the shaped article to a temperature at which the polymer crystallizes to cause phase separation to occur between the thermoplastic polymer and the compound.
U.S. Pat. No. 4,726,989 (Mrozinski) discloses microporous materials incorporating a nucleating agent made by melt blending a crystallizable thermoplastic polymer with a nucleating agent which is capable of inducing subsequent crystallization of the thermoplastic polymer and with a compound which is miscible with the thermoplastic polymer at the melting temperature of the polymer but phase separates on cooling at or below the crystallization temperature of the polymer, forming a shaped article of the melt blend, cooling the shaped article to a temperature at which the nucleating agent induces the thermoplastic polymer to crystallize so as to cause phase separation to occur between the thermoplastic polymer and the compound.